Mobile networks are currently evolving from pure circuit switched (CS) networks towards IP based networks, and by that integrating into IP based infrastructures that are also used for the Internet, the World Wide Web and the datacom industry.
With this trend mobile networks closely follow the evolution steps of the wireline networks, where Voice over Internet Protocol (VoIP) via Digital Subscriber Line (DSL) access or via Wireless Local Are Network (WLAN) access is existing technology today.
Mobile operators that install IP Multimedia Subsystem (IMS) networks and offer IMS services want to make these services available also to Global Systems for Mobile Communications (GSM)/Wideband Code Division Multiple Access (WCDMA) subscribers. Hereto, calls from and to this group of GSM/WCDMA subscribers are routed through the IMS network, in order for the IMS service engine to execute the call and call related services. This concept is called IMS Centralized Services (ICS). The IMS centralized Services work item in the 3rd Generation Partnership Program (3GPP) targets at using the CS access for access to IMS services (for example, 3GPP Technical Specification 23.292), and is complemented by IMS Service Continuity (for example, 3GPP Technical Specification 23.237).
In parallel to the ICS trend the packet core is evolving (Evolved Packet Core, EPC) as part of the Evolved Packet System (EPS), supporting EUTRAN (or LTE as it also is denoted) as new radio access network. As part of this discussion, work on Single Radio Voice Call Continuity (SRVCC) is ongoing in 3GPP SA2 (for example 3GPP Technical Specification 23.216), enabling inter domain Handover (HO) of an IMS voice call from packet switched (PS) domain to circuit switched (CS) domain. It is presumed that also the return direction might be standardized in later releases of the standard. The inter domain HO is one step of a complete session transfer in SRVCC, the second step moves the IMS session from a PS access to a CS access, this is done in the SCC AS specified in above mentioned IMS Service continuity specification.
Referring to FIG. 1, a known communications architecture used during a session or a call being anchored in a home network of a subscriber will be described in the following. The communications architecture may be known from the 3GPP Technical Specifications 23.237 and 23.216.
A subscriber UE is registered in IMS via a LTE or UTRAN HSPA access network. The call of the UE is anchored in a Service Centralization and Continuity Application Server (SCC AS) of the IMS. The access network and the IMS, in which the call is anchored, is referred to as a home network or home domain of the subscriber.
A visited network or a visited domain to which the subscriber may roam comprises either a GERAN or UTRAN access network and a LTE or UTRAN HSPA access network and a core network comprising at least one Mobile Switching Center Server (MSC-S) and at least one Media Gateway (MGW).
Call control signaling is carried from the visited domain via the eNodeB and Serving Gateway (S-GW) and PDN-GW to the IMS (which comprises a P-CSCF, a S-CSCF and a SCC AS at least) and vice versa. The media is transported via the eNodeB and Serving Gateway (S-GW) and PDN-GW to the remote end, whereby the remote end is an IMS terminal or any other media handling device e.g. a Media Resource Function Processor (MRFP), and vice versa.
However, the current SRVCC solution foresees that the SCC AS, which is the mobility anchor point for the session from a IMS control plane point of view, is in the home domain, also when the subscriber is roaming. The media anchor is really the remote end. This kind of communications architecture may results in a bad se ice experience which may be deduced from the following:
During the session transfer a SIP signaling carrying SDP swill have to be routed back to the home network and then all the way to the remote end in order for the media path to change. This is done by means of changing the ports on the media plane.
A possible solution to remedy this problem may be that the SCC AS inserts a Multimedia Resource Function Control (MRFC) in the home IMS network. The MRFC will insert a MRFP in the media path, in order to minimize a time delay which is introduced by updating the remote end as described previous (particular information as to a further subscriber being called), however, also in this case the SCC AS the MRFC and MRFP would be in the home network of the subscriber.
It may be worth noted that it is not possible to use an IMS entity of a visited network for this purpose; the subscriber has to use the IMS of the home network.